Integrated circuit wireless communication unit and method for providing a power supply

ABSTRACT

An integrated circuit comprises a radio frequency (RF) power amplifier (PA) output stage; at least one amplifier stage prior to the RF PA output stage; a linear amplifier comprising a voltage feedback wherein the linear amplifier is operably coupled to a low frequency supply module such that the linear amplifier and low frequency supply module provide a combined first power supply to the RF PA output stage; and a switched mode power supply module arranged to provide a second power supply to the linear amplifier and to the at least one amplifier stage prior to the RF PA output stage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/345,760 (filed on Jan. 9, 2012), which claims the benefit of U.S.provisional application No. 61/438,347 (filed on Feb. 1, 2011) and U.S.provisional application No. 61/563,316 (filed on Nov. 23, 2011). Theentire contents of these related applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of this invention relates to wireless communication units,transmitter architectures and circuits for providing a power supply. Theinvention is applicable to, but not limited to, power supply integratedcircuits for linear transmitter and wireless communication units and apower amplifier supply voltage method therefor.

2. Description of the Prior Art

A primary focus and application of the present invention is the field ofradio frequency (RF) power amplifiers capable of use in wirelesstelecommunication applications.

Continuing pressure on the limited spectrum available for radiocommunication systems is forcing the development of spectrally-efficientlinear modulation schemes. Since the envelopes of a number of theselinear modulation schemes fluctuate, these result in the average powerdelivered to the antenna being significantly lower than the maximumpower, leading to poor efficiency of the power amplifier. Specifically,in this field, there has been a significant amount of research effort indeveloping high efficiency topologies capable of providing highperformances in the ‘back-off’ (linear) region of the power amplifier.

Linear modulation schemes require linear amplification of the modulatedsignal in order to minimise undesired out-of-band emissions fromspectral re-growth. However, the active devices used within a typical RFamplifying device are inherently non-linear by nature. Only when a smallportion of the consumed DC power is transformed into RF power, can thetransfer function of the amplifying device be approximated by a straightline, i.e. as in an ideal linear amplifier case. This mode of operationprovides a low efficiency of DC to RF power conversion, which isunacceptable for portable (subscriber) wireless communication units.Furthermore, the low efficiency is also recognised as being problematicfor the base stations.

Furthermore, the emphasis in portable (subscriber) equipment is toincrease battery life. To achieve both linearity and efficiency, socalled linearisation techniques are used to improve the linearity of themore efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’amplifiers. A number and variety of linearising techniques exist, whichare often used in designing linear transmitters, such as CartesianFeedback, Feed-forward, and Adaptive Pre-distortion.

Voltages at the output of the linear, e.g. Class AB, amplifier aretypically set by the requirements of the final RF power amplifier (PA)device. Generally, the minimum voltage of the PA is significantly largerthan that required by the output devices of the Class AB amplifier.Hence, they are not the most efficient of amplification techniques. Theefficiency of the transmitter (primarily the PA) is determined by thevoltage across the output devices, as well as any excess voltage acrossany pull-down device components due to the minimum supply voltage (Vmin)requirement of the PA.

In order to increase the bit rate used in transmit uplink communicationchannels, larger constellation modulation schemes, with an amplitudemodulation (AM) component are being investigated and, indeed, becomingrequired. These modulation schemes, such as sixteen-bit quadratureamplitude modulation (16-QAM), require linear PAs and are associatedwith high ‘crest’ factors (i.e. a degree of fluctuation) of themodulation envelope waveform. This is in contrast to the previouslyoften-used constant envelope modulation schemes and can result insignificant reduction in power efficiency and linearity.

To help overcome such efficiency and linearity issues a number ofsolutions have been proposed. One technique used relates to modulatingthe PA supply voltage to match the envelope of the radio frequencywaveform being transmitted by the RF PA. Envelope modulation requires afeedback signal from the PA supply to one of the control ports of theamplifier. Proposed solutions that utilise envelope modulation includeenvelope elimination and restoration (EER), and envelope tracking (ET).Both of these approaches require the application of a wideband supplysignal to the supply port of the PA.

It is known that the use of PA supply RF envelope tracking may improveboth PA efficiency and linearity for high peak-to-average power (PAPR)high power transmit conditions. FIG. 1 illustrates a graphicalrepresentation 100 of two alternative techniques; a first technique thatprovides a fixed voltage supply 105 to a PA, and a second techniquewhereby the PA supply voltage is modulated to track the RF envelopewaveform 115. In the fixed supply case, excess PA supply voltageheadroom 110 is used (and thereby potentially wasted), irrespective ofthe nature of the modulated RF waveform being amplified. However, forexample in the PA supply voltage tracking of the RF modulated envelopecase 115, excess PA supply voltage headroom can be reduced 120 bymodulating the RF PA supply, thereby enabling the PA supply toaccurately track the instant RF envelope.

It is known that switched-mode power supply (SMPS) techniques may beused to provide improved efficiency. A SMPS is an electronic powersupply that incorporates a switching regulator in order to be highlyefficient in the conversion of electrical power. Like other types ofpower supplies, an SMPS transfers power from a source, such as a batteryof a wireless communication unit, to a load, such as a power amplifiermodule, whilst converting voltage and current characteristics. An SMPSis usually employed to efficiently provide a regulated output voltage,typically at a level different from the input voltage. Unlike a linearpower supply, the pass transistor of a switching mode supply switchesvery quickly between full-on and full-off states, which minimize wastedenergy. Voltage regulation is provided by varying the ratio of ‘on’ to‘off’ time. In contrast, a linear power supply must dissipate the excessvoltage to regulate the output. This higher efficiency is the primaryadvantage of a switched-mode power supply. Switching regulators are usedas replacements for the linear regulators when higher efficiency,smaller size or lighter weight power supplies are required. They are,however, more complicated, their switching currents can cause electricalnoise problems if not carefully suppressed, and simple designs may havea poor power factor.

FIG. 2 illustrates graphically 200 output power 205 versus input power210, various functional and operational advantages that can be achievedwhen a PA supply (drain) voltage is modulated to use an envelopetracking technique. By enabling the PA (drain) supply voltage to trackthe instant RF envelope 115, the PA may be kept in modest compression atconstant gain 215 over the range of the amplitude modulation toamplitude modulation (AM-AM) curves 220. Such tracking of the supplyvoltage of the instant RF envelope 115 enables a higher output powercapability 225 for the same linearity (using envelope tracking) to beachieved by the transmitter, as compared to techniques that do not allowthe PA supply voltage to track the instant RF envelope of the PA. Inaddition, the envelope tracking graph 200 may also be viewed as beingable to support a PA gain reduction when employing ET 230, as comparedto an architecture that considers PA gain with a fixed supply. A skilledartisan will appreciate that this is predominantly a consequence of PAcharacteristics together with a function of the operation point of thePA under the chosen operating conditions for envelope tracking.

Thus, and advantageously, the gain of the PA that may be achieved whenenvelope tracking is implemented may be reduced 230 as compared to thePA gain that uses a fixed PA supply voltage. Envelope tracking may alsosupport a high efficiency gain potential for high PAPR conditions. Inaddition, the PA may operate at a cooler temperature for the same outputpower, thereby reducing heat loss and increasing efficiency. However, itis also known that envelope tracking requires a high efficiency, highbandwidth supply modulator and accurate tracking of the RF envelope istherefore difficult to achieve in practical implementations.

FIG. 3 illustrates graphically 300 envelope spectral density 305 versusfrequency 310 required when a PA supply (drain) voltage is modulatedusing an envelope tracking technique. FIG. 3 further illustratesgraphically 350 a corresponding integrated amplitude modulated power 355versus frequency 360. Envelope spectral density exhibits a number ofcommon features for different modulation cases, for example, alow-frequency region, which contains the majority of the energy, and ahigh-frequency region, which must be reproduced up to, say, 4-8 MHz. Asillustrated, the two energy regions are separated by a region, coveringa range of roughly 10 kHz-400 kHz, which contains little energy.

Thus, a need exists for improved power supply integrated circuits,wireless communication units and methods for power amplifier supplyvoltage control that use such linear and efficient transmitterarchitectures, and in particular a wideband power supply architecturethat can provide a supply voltage in a power efficient manner.

SUMMARY OF THE INVENTION

Accordingly, the invention seeks to mitigate, alleviate or eliminate oneor more of the above mentioned disadvantages, either singly or in anycombination. Aspects of the invention provide an integrated circuit, awireless communication unit and a method for providing a switch modepower supply as described in the appended claims.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, aspects and embodiments of the invention will bedescribed, byway of example only, with reference to the drawings.Elements in the figures are illustrated for simplicity and clarity andhave not necessarily been drawn to scale. Like reference numerals havebeen included in the respective drawings to ease understanding.

FIG. 1 illustrates a graphical representation of a first power supplytechnique that provides a fixed voltage supply to a PA, and a secondpower supply technique whereby the PA supply voltage is modulated totrack the RF envelope.

FIG. 2 illustrates graphically various functional and operationaladvantages that can be achieved when a PA supply (drain) voltage ismodulated to use an envelope tracking technique.

FIG. 3 illustrates graphically a power spectral density versus frequencywhen a PA supply (drain) voltage is modulated to use an envelopetracking technique.

FIG. 4 illustrates an example block diagram of a wireless communicationunit adapted to support envelope tracking.

FIG. 5 illustrates one example block diagram of a part of a power supplycircuit of a transmitter chain of a wireless communication unit adaptedto support envelope tracking.

FIG. 6 illustrates a further example block diagram of a part of a powersupply circuit of a transmitter chain of a wireless communication unitadapted to support envelope tracking.

FIG. 7 illustrates an example timing diagram of a power supply circuitof a transmitter chain of a wireless communication unit adapted tosupport both envelope tracking and fixed drain.

FIG. 8 illustrates a yet further example block diagram of a part of apower supply circuit of a transmitter chain of a wireless communicationunit adapted to support envelope tracking.

FIG. 9 illustrates a still further example block diagram of a part of apower supply circuit of a transmitter chain of a wireless communicationunit adapted to support envelope tracking.

FIG. 10 illustrates a yet still further example block diagram of a partof a power supply circuit of a transmitter chain of a wirelesscommunication unit adapted to support envelope tracking.

FIG. 11 illustrates a yet still even further example block diagram of apart of a power supply circuit of a transmitter chain of a wirelesscommunication unit adapted to support envelope tracking.

FIG. 12 illustrates an example flowchart for envelope tracking.

FIG. 13 illustrates an example block diagram of a part of a poweramplifier circuit of a transmitter chain of a wireless communicationunit with different power stages fed from independent power supplies.

FIG. 14 illustrates a yet further example block diagram of a part of apower amplifier circuit of a transmitter chain of a wirelesscommunication unit adapted to support both envelope tracking and fixeddrain modes of operation.

FIG. 15 illustrates an example block diagram of a part of a power supplycircuit of a transmitter chain of a wireless communication unit adaptedto support both envelope tracking and fixed drain modes of operation.

FIG. 16 illustrates a typical computing system that may be employed toimplement signal processing functionality in embodiments of theinvention.

DETAILED DESCRIPTION

Examples of the invention will be described in terms of one or moreintegrated circuits for use in a wireless communication unit, such asuser equipment in third generation partnership project (3GPP™) parlance.However, it will be appreciated by a skilled artisan that the inventiveconcept herein described may be embodied in any type of integratedcircuit, wireless communication unit or wireless transmitter that couldbenefit from improved linearity and efficiency. In some describedexamples of the invention, a power supply for a power amplifier, forexample as part of a linear transmitter, has been adapted to support awideband power supply that may provide improved linearity and efficiencyto an RF PA. Although examples of the invention have been described withrespect to an envelope tracking design, it is envisaged that theinvention may be implemented in any transmitter architecture.

Furthermore, although examples of the invention have been described withrespect to transmission of predominantly amplitude modulated waveforms,it is envisaged that the invention may be implemented with any waveformstructures, particularly where the majority of the energy is located atfrequencies close to DC.

In addition, although examples of the invention have been described withrespect to a wideband linear transmitter architecture, as the efficiencybenefits are most relevant to wideband systems with specific propertiesthat allow the benefits of using efficient switch mode power supplies tosupply much of the energy to be realised, it is envisaged that theinvention may be also implemented in a narrowband linear transmitterarchitecture, such as Cartesian feedback or adaptive pre-distortion.

In some examples of the invention, a number of control mechanisms is/areprovided in order to optimise a DC level of a linear amplifier (e.g.class AB amplifier) output that is used in conjunction with a switchmode power supply for a radio frequency power amplifier. With knownenvelope modulated/envelope tracking systems, the crest factor (peak toaverage ratio (PAR)) of the envelope waveform may exceed 3 dB, whereas atarget amplifier output voltage setting would be of the order of lessthan VDD/2. In some examples of the invention, the control mechanismsdescribed may have minimal or no additional overhead on current suppliedby the linear amplifier (e.g. class AB) output. Furthermore, in someexamples of the invention, the control mechanisms may have minimal or noeffect on the switch mode power supply to the radio frequency poweramplifier.

An architecture is described for providing a modulator supply, which inone example is a composite/hybrid supply comprising a switch mode and/orlower frequency part and a linear and/or higher frequency part to aradio frequency (RF) power amplifier (PA). The integrated circuitcomprises a low-frequency power supply path comprising a switchingregulator and a high-frequency power supply path, whereby in combinationthe low-frequency power supply path and high-frequency power supply pathprovide a power supply to an output port of the integrated circuit forcoupling to a load, such as in one example a supply port of the RF PA.The architecture, which in some examples may comprise one or moreintegrated circuits and/or components, further comprises an amplifiercore arranged to drive a power supply signal on the high-frequency powersupply path wherein the amplifier core comprises an input comprising avoltage feedback from the output port. In some examples, a switch modepower supply (SMPS) acts as a controlled current source where thevoltage feedback provides control of the SMPS voltage. Also this voltagefeedback loop ensures that the voltage at the load (e.g. supply port ofthe RF PA), which is a composite of the instantaneous currents from theswitched mode power supply and the amplifier interacting with theimpedance of the load (e.g. supply port of the PA), tracks the targetreference voltage. The amplifier output is AC coupled to the powersupply path. Hence, in this manner and in some example embodiments, anintegrated circuit for providing an improved linear and efficient supplyvoltage for a power amplifier, and in particular a wideband power supplyvoltage for a power amplifier, is described.

Referring first to FIG. 4, a block diagram of a wireless communicationunit (sometimes referred to as a mobile subscriber unit (MS) in thecontext of cellular communications or an user equipment (UE) in terms ofa 3^(rd) generation partnership project (3GPP™) communication system) isshown, in accordance with one example embodiment of the invention. Thewireless communication unit 400 contains an antenna 402 preferablycoupled to a duplex filter or antenna switch 404 that provides isolationbetween receive and transmit chains within the wireless communicationunit 400.

The receiver chain 410, as known in the art, includes receiver front-endcircuitry 406 (effectively providing reception, filtering andintermediate or base-band frequency conversion). The front-end circuitry406 is coupled to a signal processing function 408. An output from thesignal processing function 408 is provided to a suitable user interface430, which may encompass a screen or flat panel display. A controller414 maintains overall subscriber unit control and is coupled to thereceiver front-end circuitry 406 and the signal processing function 408(generally realised by a digital signal processor (DSP)). The controlleris also coupled to a memory device 416 that selectively stores variousoperating regimes, such as decoding/encoding functions, synchronisationpatterns, code sequences, and the like.

In accordance with examples of the invention, the memory device 416stores modulation data, and power supply data for use in supply voltagecontrol to track the envelope of the radio frequency waveform output bythe wireless communication unit 400 and processed by signal processingfunction 408. Furthermore, a timer 418 is operably coupled to thecontroller 414 to control the timing of operations (transmission orreception of time-dependent signals and in a transmit sense the timedomain variation of the PA (drain) supply voltage within the wirelesscommunication unit 400).

As regards the transmit chain, this essentially includes the userinterface 430, which may encompass a keypad or touch screen, coupled inseries via signal processing function 428 to transmitter/modulationcircuitry 422. The transmitter/modulation circuitry 422 processes inputsignals for transmission and modulates and up-converts these signals toa radio frequency (RF) signal for amplifying in the power amplifiermodule or integrated circuit 424. RF signals amplified by the PA moduleor PA integrated circuit 424 are passed to the antenna 402. Thetransmitter/modulation circuitry 422, power amplifier 424 and PA supplyvoltage module 425 are each operationally responsive to the controller414, with the PA supply voltage module 425 additionally responding to areproduction of the envelope modulated waveform from thetransmitter/modulation circuitry 422.

The signal processor function 428 in the transmit chain may beimplemented as distinct from the processor 408 in the receive chain 410.Alternatively, a single processor may be used to implement processing ofboth transmit and receive signals, as shown in FIG. 4. Clearly, thevarious components within the wireless communication unit 400 can berealised in discrete or integrated component form, with an ultimatestructure therefore being merely an application-specific or designselection.

Furthermore, in accordance with examples of the invention, thetransmitter/modulation circuitry 422, together with power amplifier 424,PA supply voltage 425, memory device 416, timer function 418 andcontroller 414 have been adapted to generate a power supply to beapplied to the PA 424. For example, a power supply is generated that issuitable for a wideband linear power amplifier, and configured to trackthe envelope waveform applied to the PA 424.

Referring now to FIG. 5, one generic example block diagram of a part ofa power supply circuit 500 of a transmitter chain of a wirelesscommunication unit is illustrated, for example the wirelesscommunication unit of FIG. 4. The power supply circuit 500 in FIG. 5 hasbeen configured and/or adapted to support envelope tracking. A poweramplifier (PA) 424 receives an envelope modulated RF signal 502 as aninput RF signal to be amplified. The PA 424 amplifies the RF signal andoutputs an amplified envelope modulated RF signal to an antenna 402. ThePA 424 receives a power supply from a power supply integrated circuit520, as illustrated. A power source, such as battery 508, is operablycoupled to a low-frequency-path supply module 518 in the power supplyintegrated circuit 520, which in one example is arranged to supply alow-frequency current 534, as part of a power supply to the PA 424, in ahighly efficient manner.

The battery 508 is also operably coupled to a high-frequency-path supplymodule 506, which in one example is arranged to provide a voltagesupply, such as a switch mode power supply, to a linear amplifier 504 ina highly efficient manner. In an alternative example, thehigh-frequency-path supply module 506 may be by-passed, such that thelinear amplifier 504 is supplied directly from the power source, e.g.battery 508. The linear amplifier 504 receives, as a first input, anenvelope signal 503 that is arranged to track the envelope of the RFsignal 502 that is input to the PA 424. The linear amplifier 504comprises a second input that receives voltage feedback 510 of thevoltage 528 applied to the PA 424, which is used to control the voltageat the load (e.g. power supply port of the PA 424).

The low-frequency-path supply module 518 receives, as an input, avoltage feedback signal 514 coupled from the output 512 of the linearamplifier 504. The output 512 from the linear amplifier 504 is alsocoupled to the voltage at the power supply port of the PA 424 through acapacitor 533. The linear amplifier 504, which in one example is of aclass-AB configuration, provides power supply signal energy to an outputof the power supply IC 520 that is not supplied by the low-frequencysupply module 518.

In one example circuit, within the low-frequency-path supply module 518there exists an error amplifier 529. The error amplifier 529 comparesthe voltage feedback signal 514 to a reference voltage 530, and producesan error voltage 531. In some examples, the error amplifier 529 alsoincludes frequency compensation to ensure stability of the feedbackloop. In one possible example, the frequency compensation may have anintegrating characteristic, such that the time-averaged differencebetween the reference voltage 530 and the sense voltage 514 is driven tozero. The unity-gain bandwidth of the integrator may be constrained tobe lower in frequency than other dynamic elements of the feedback loop,so as to ensure stability. In alternative examples, it is envisaged thatother frequency compensation techniques used in switching regulators mayalso be used. In this manner, the error voltage 531 acts as an input toa pulsewidth modulator 532, which provides a low-frequency current 534to the inductor 515. This arrangement is commonly used in switchingregulators. In one example, the pulsewidth modulator 532 operates bycomparing the error voltage 531 to a periodic triangular waveform offixed ramp rate. The output of this comparison is a pulsewidth-modulatedsignal that can be used to generate the low-frequency current 534.

In a steady state condition, the low-frequency current 534 that isapplied to the power supply port of the PA 424 may be arranged to besufficient to provide the DC current, whilst the linear amplifier 504sources the AC current. In this manner, the use of a voltage sensearrangement, as described, facilitates monitoring the output voltage 512of the linear amplifier 504. The low-frequency-path supply module actsto maintain the output voltage 512 of the linear amplifier at such alevel that the amplifier operates within its designed output voltagerange. It does so by varying the level of output current 534 provided.Current sensing may be used in some examples to improve the response ofthe switching regulator. Hence, the linear amplifier 504 is suppliedfrom a second switch mode power supply (SMPS), namely thehigh-frequency-path supply module 506, with the output of the linearamplifier 504 AC coupled (via the high-frequency path coupling element533) to the output feeding the load (namely the power supply port of thePA 424).

Advantageously, AC coupling of the high-frequency power supply signal tothe output port of the IC 520 using the coupling capacitor 533 allowsthe quiescent voltage operating point at the output of the linearamplifier 504 to be decoupled from the supply requirements of the poweramplifier 424, thereby taking advantage of the differences in thevoltage compliance requirements of the linear amplifier 504 and thepower amplifier 424.

In order to better appreciate the operation of FIG. 5, let us considerthat the ac-coupling capacitor 533 stores a fixed charge, resulting in afixed voltage Vcap and that the low-frequency path is inactive. If thelinear amplifier 504 has an output voltage Vamp, then the supply voltageto the PA will be Vamp+Vcap. If the output voltage of the linearamplifier 504 has an average value Vampdc and a time-varying valueVampac, then the supply voltage to the PA 424 will be:

Vampdc+Vampac+Vcap.

Thus the average value of the supply voltage to the PA 424 will beVcap+Vampdc, its maximum value will be Vcap+Vampdc plus the maximumvalue of Vampac, and its minimum value will be Vcap+Vampdc plus theminimum value of Vampac.

By selecting an appropriate value for the level shifting voltage Vcap,it is possible to reduce the supply voltage to the linear amplifier 504such that it is just sufficient to supply a full range of an ac voltageswing on the PA supply, as well as allow the capacitor 533 add enoughvoltage to provide the correct average voltage on the PA supply.Minimizing the supply voltage to the linear amplifier in this way alsominimizes power dissipation in the supply modulator 520. For this schemeto function properly in a real circuit, the LF supply 518 should beconfigured to maintain the proper voltage across the ac couplingcapacitor. Further examples are illustrated in later figures detailinghow this can be achieved.

The example in FIG. 5 uses a control loop, sensing the voltage at theoutput voltage of the linear amplifier 504, to control the current ofthe main SMPS. In this example arrangement, the voltage across the ACcoupling capacitor is determined by the voltage sense at the output ofthe linear amplifier 504, which is then compared with a target voltage,together with a voltage sensed at the PA load, which is fed back to the(differential) linear amplifier 504 where it is compared with theenvelope reference signal 503.

When a SMPS is used to supply the linear amplifier 504, this reducedvoltage supply requirement for the linear amplifier 504 requires a lowercurrent draw from the main energy source e.g. battery 508, as comparedto a case when it is directly coupled to the PA load, thereby resultingin an overall efficiency improvement. In some examples, andadvantageously, the target amplifier output voltage can be adjusted fordifferent output power levels and transmission modulation schemes, inorder to optimise the amplifier supply requirements.

The generic example block diagram of FIG. 5 comprises at least thefollowing common circuit elements or components that are replicated inthe example embodiments of FIGS. 6-10: a low-frequency power pathimplemented for example with a switching regulator; a high-frequencypower supply path driven by an amplifier, such as a linear amplifier 504exhibits, say, a Class AB mode of operation; voltage feedback 514 fromthe output of the linear amplifier 504 to the switching regulator of thelow-frequency power supply path 518; a voltage feedback 510 from the PAsupply voltage 528 to the linear (e.g. Class AB) amplifier 504; and acapacitor 533 (and in some examples inductor 515) that couples thehigh-frequency and low-frequency power supply paths together. By usingthe capacitor 533 (and in some examples inductor 515), it is possible tocombine power at dc and low frequencies from the switching regulator 518with higher-frequency ac power from the linear amplifier 504.

Thus, FIG. 5 illustrates a means for implementing a wideband powersupply in a power efficient manner. The power supply is configured toprovide power to a load, such as a power supply for a RF Power Amplifier(PA), and in particular an envelope tracking supply that may achievehigh efficiency when driving PAs of differing load characteristics. Thepower supply may also, and advantageously, be configured to providesupply envelopes corresponding to different modulation formats.

Referring now to FIG. 6, a more detailed example block diagram of a partof a power supply circuit 600 of a transmitter chain of a wirelesscommunication unit, adapted to support envelope tracking, isillustrated. The operation of elements described with reference to FIG.5, with like reference numerals used, is not replicated in describingFIG. 6 in order to ease understanding. The battery andhigh-frequency-path supply are used in the same way in FIG. 6 as in FIG.5, but their symbols have been removed from FIG. 6 for clarity. In FIG.6, an envelope voltage 602 is input to an envelope conditioning module603. The envelope conditioning module 603 is arranged to modify andlimit the envelope signal characteristics, which in some examples mayinvolve one or more of a number of actions, for example:

-   -   (i) limiting a minimum value of the power supply to meet        requirements of the PA    -   (ii) reducing the peak-to-peak voltage of the envelope signal        improving efficiency,    -   (iii) restricting the signal bandwidth of the envelope signal,    -   (iv) performing any necessary gain and offset alignment of the        envelope signal; and    -   (v) implementing any signal formatting, such as converting        between differential and single ended representation.

The inventors have identified that an envelope tracking supply haslimited benefits when lower output levels are used, or certainmodulation schemes are used with reduced AC content leading to lower PARenvelope waveforms. For such low output levels and/or modulationschemes, the DC voltage applied to the PA has a greater significancewith the power of the AC content of the envelope significantly reduced,negating the benefits of envelope tracking and the efficiencyperformance gain is reduced. Therefore, in these scenarios, a fixeddrain (FD) mode of operation is able to take full advantage of the fullswitching supply. Although the application of the DC components and ACcomponents is described with reference to FIG. 6, it is envisaged thatsuch application is common to a number of the other described exampleembodiments.

Thus, in some examples of the invention, two modes of operation aresupported, namely an envelope tracking (ET) mode and a fixed drain (FD)mode. The selection of the mode to be used in providing the power supplyto the PA is performed by mode control module 616.

ET Mode:

In ET mode, the PA power supply is a time varying signal, which tracksthe required signal envelope, in order to achieve the efficiencybenefits discussed.

There are at least two operational factors that favour use of ET mode,namely high crest factor signals (i.e. where the peak-to-average ratioof the envelope signal is high) and higher output power levels, whereasthe minimum voltage requirement of the PA and the power overhead of theAC path (including the quiescent power of the amplifier) is lessimportant. Hence, in one example, the benefit of the mode control module616 setting the power supply to an ET mode of operation is greatest forsignals using modulation schemes that result in high crest factor powerenvelope signals, for the upper section of the output power range.

In both the ET mode and the FD mode, the power supply system has toprovide the full power spectrum, i.e. both high-frequency andlow-frequency energy. Both the ET mode and the FD mode use a switch modepower supply (SMPS) arrangement in order to supply the low-frequencypower. However, the two modes of operation differ in how they handle thehigh-frequency requirements.

In ET mode, the switch 614 is configured as ‘open’, the linear amplifier504 and high-frequency-path supply module 506 are enabled, and the (ET-)sense feedback input 514 for the low-frequency-path supply module 518 isselected. The linear amplifier 504 operates in voltage feedback to forcethe output voltage 528 to be substantially equal to the conditionedenvelope voltage 503. The ET-sense feedback voltage 514 may then becompared to a reference voltage 530, which in one example is generatedusing a digital-to-analog converter (DAC) (not shown).

Thus, in this manner in the ET mode, the AC coupling capacitor 533performs a function of a dc level shifter and the high-frequency poweris provided by the linear amplifier 504. Using this active path for thehigh-frequency power allows the output power supply to track the RFenvelope (see FIG. 1 image 120), which limits the power dissipated inthe PA 424. However, in ET mode, the power dissipation in the powersupply module may be larger because the high-frequency-path power supply506 and the linear amplifier 504 must be powered on.

In ET mode, the circuit of FIG. 6 may be operated such that there isalways a positive charge, for example the voltage at the output 528 isgreater than voltage at the output of the amplifier 512, stored on thecapacitor 533. In this manner, it is possible for the output voltage 528to exceed the output range of the linear amplifier 504. The power supplyproduced by the high-frequency-path power supply 506 need be only ofsufficient voltage to sustain the ac amplitude of the envelope voltage602. In this way, the power dissipated within the linear amplifier canbe minimized.

The voltage feedback loop, which includes the inverting stage 625 andthe low-frequency-path power supply 518, ensures that the average outputvoltage of the linear amplifier 504 and the voltage across the couplingcapacitor 533 are maintained at the appropriate levels. The invertingstage 625 produces a complementary signal to the linear amplifier outputvoltage 512. The feedback voltage 514 is then passed through the analogmultiplexer 628, which is used to select between ET and FD modes. Theoutput of the analog multiplexer 628 is then compared to a referencevoltage 530 in error amplifier 529, and the resulting error voltage Verr531 is generated.

As in FD mode, the error amplifier 529 contains compensation tostabilize the loop. The compensation has a low-pass characteristic,which helps to filter out high-frequency information present in thefeedback voltage 514. Also as in FD mode, the pulsewidth modulatorformed by the comparator 630 and ramp voltage 631 produces a pulsewidthmodulated power output 627. This power output is filtered by inductor622 to provide a roughly constant current to the output 528. In someexamples, the inductor 622 and capacitor 533 form a low-pass filter,which is configured to locate a double pole in a low energy range of apower spectral density of the reference signal 530. In this manner, thefeedback loop acts in such a way as to maintain the average amplifieroutput voltage 512 equal to the reference voltage REF 530.

FD Mode:

In FD mode, switch 614 is configured as ‘closed’, the linear amplifier504 and high-frequency-path supply module 506 are disabled, and the(FD-) sense feedback input 629 of the low-frequency-path power supply isselected. In FD mode, the output 512 from the linear amplifier 504 iscoupled to a fixed drain (FD) mode switch 614, which in a closedconfiguration (as set, for example by mode control module 616) groundsthe output from the linear amplifier 504. In this FD mode, the PA powersupply is fixed at the minimum voltage requirement (of the PA 424) inorder to support the transmitted envelope waveform, for example for atime period between power level updates.

In the FD mode, the power supply may be re-configured to use the ACcoupling capacitor 533 as a filtering element for the DC-DC SMPS. Inthis manner, the AC coupling capacitor 533 provides the high-frequencypower required by the PA. In FD mode, the linear amplifier andhigh-frequency-path regulator may be disabled to save power. The PAsupply voltage 528 is at a higher level in FD mode (see FIG. 1 image110), but the quiescent current of the power supply is lower.

In some examples, the operation of the circuit in FD mode resembles aconventional voltage-mode buck regulator. The FD-sense feedback voltage629 passes through an analog multiplexer. The FD-sense feedback voltage629 is then compared to a reference voltage REF 530, which in oneexample is generated using a digital-to-analog converter (DAC) (notshown). The difference between the FD-sense feedback voltage 629 andvoltage REF 530 is amplified by the differential error amplifier 529,which includes frequency compensation for loop stability. The resultingerror voltage Verr 531 is compared to a ramp voltage 631 by comparator630.

In some examples, the comparator 630 may be reset at, say, a fixedperiodic rate by a clock signal, thereby producing a rising edge at afixed rate, whilst the falling edge is determined, for example, by theoutput of the comparator 630, thereby producing a pulse wave modulated(PWM) power output 627. In some examples, the PWM power output may thenbe filtered by, say, inductor 622 and coupling capacitor 533 in order toremove high frequency components and produce the output power supply528. The feedback loop acts to maintain the voltage at the output 528that may be equal to the input reference signal REF 530. Thisconfiguration is commonly used in switched-mode power supplies and isknown as ‘voltage-mode’ control.

As an alternative to PWM modulation, any of a number of well-knownmodulation schemes that convert a control voltage to a duty cycle may beused.

Transition Between ET Mode and FD Mode:

A particular feature of the more detailed example block diagram of apart of a power supply circuit 600 of FIG. 6 is that it supports atransition from FD mode in a first (e.g. n−1) time slot 705 to ET modein a second (e.g. n) time slot 710 and thereafter from ET mode in thesecond (e.g. n) time slot 710 to FD mode in a third (e.g. n+1) time slot715, as illustrated in the example timing diagram 700 of FIG. 7. Thearchitecture shown in FIG. 6 ensures a speedy transition with a minimumof disruption to the power supply output 528. The same error voltageVerr 531 is used in both FD mode and in ET mode, which ensures that noabrupt changes in duty cycle are observed during a mode transition. Whentransitioning from FD mode in a first (e.g. n−1) time slot 705 to ETmode in a second (e.g. n) time slot 710, it is desirable that theamplifier output 512 of linear amplifier 504 makes a gradual transitionfrom ground to its final modulated voltage. In order to ensure this,during the transition from FD mode in a first (e.g. n−1) time slot 705to ET mode in a second (e.g. n) time slot 710, the dc and ac values ofthe conditioned envelope voltage 503 are gradually ramped up from zerovolts 720 to their final values 725. Conversely, when making atransition from ET mode in the second (e.g. n) time slot 710 to FD modein a third (e.g. n+1) time slot 715, the dc and ac values of theconditioned envelope voltage 503 are gradually decreased from theirsteady-state values 725 to zero volts 720. Through use of such rampedand tapered envelope signals, abrupt disturbances to the control loopcan be eliminated, thereby keeping the power supply within regulationthroughout the transition, as illustrated in FIG. 7

FIG. 8 illustrates a yet further example block diagram 800 of a part ofa power supply circuit of a transmitter chain of a wirelesscommunication unit adapted to support envelope tracking. The yet furtherexample of FIG. 8 highlights an alternative way of controlling thelow-frequency-path supply 518. For ease of understanding, and not toobfuscate or distract from the description of FIG. 8, electroniccomponents and circuits of the transmitter chain described withreference to earlier figures will not be explained again in any greaterextent than that considered necessary.

It is contemplated that any of a number of control methods may be usedin the low-frequency-path power supply 518, both in FD mode and in ETmode in various example embodiments of the invention. For example, thewell-known voltage-mode control approach may be used, as shown in FIG.6. In the example of FIG. 8, a current-mode feedback control loopincluding elements 801, 802 and 630 is used. Current-mode control is awell-known method used in switching regulators, in which the currentthrough the inductor is sensed and feedback control applied to stabilizeit. A voltage feedback loop is also used to regulate the output voltage,just as in a voltage-mode controller. The ET voltage feedback loopconsists of elements 512, 625, 514, 628, 529, 531, 630, 622. The FDvoltage feedback loop includes elements 629, 628, 529, 531, 630, 622.The advantages of adding the current loop include simpler compensationmethods required for the voltage loop as well as a faster response tocertain types of transient disturbances.

In the example of FIG. 8, a conventional current-mode switched modepower supply is used as the low-frequency-path supply. A current sensor801 monitors the instantaneous current in the inductor 622. The currentis converted to a voltage by the current-to-voltage converter 802. Theresulting voltage is used as the ramp voltage 631, which is compared tothe error voltage 531 by the comparator 630. This feedback loop operatesin both FD and ET modes, just as in the circuit of FIG. 6.

FIG. 9 illustrates a still further example block diagram 900 of a partof a power supply circuit of a transmitter chain of a wirelesscommunication unit adapted to support envelope tracking. The yet furtherexample of FIG. 9 highlights an alternate method of realizingcurrent-mode feedback. For ease of understanding, and not to obfuscateor distract from the description of FIG. 9, electronic components andcircuits of the transmitter chain described with reference to earlierfigures will not be explained in any greater extent than that considerednecessary.

In the circuit of FIG. 9, the low-frequency-path power supply 518 hastwo current-sense feedback inputs. In addition to the inductor currentsensor 801 there is a current sensor 901 at the output of the linearamplifier 504. A second analog multiplexer 902 is arranged to selectbetween the two current-sense feedback inputs and passes the selectedinput through to the current-to-voltage converter 802. The current-sense801 from the low-frequency-path supply is used in FD mode, as in FIG. 8.In ET mode, the amplifier current sensor 901 is used. The output currentof the linear amplifier 504 contains high-frequency information aboutthe instantaneous current drawn by the PA 424, because the amplifiersupplies this high-frequency current. Using this configuration,information about the current demands of the PA 424 can be fed backthrough the current loop of the SMPS, which has much higher bandwidththan the voltage loop. This implies that, using the circuit of FIG. 9,the low-frequency-path voltage regulator can potentially respond fasterto the demands of the PA.

FIG. 10 illustrates a yet still further example block diagram 1000 of apart of a power supply circuit of for a PA of a transmitter chain of awireless communication unit adapted to support envelope tracking. Forease of understanding, and not to obfuscate or distract from thedescription of FIG. 10, electronic components and circuits of thetransmitter chain described with reference to earlier figures will notbe explained in any greater extent than that considered necessary.

The output 503 from the envelope conditioning module 603 is input to thelinear amplifier 1004. In FIG. 10, the devices that comprise the outputstage of the linear amplifier 1004 are shown explicitly as n-channeltransistor 1001 and p-channel transistor 1002. This example of FIG. 10does not include a discrete switch to ground the bottom plate of thecoupling capacitor 533 in FD mode. Instead, the output devices (namelyn-channel transistor 1001 and p-channel transistor 1002) of the linearamplifier 1004 can be configured in two ways. In ET mode, these twotransistors/devices operate as part of the linear amplifier 1004. In FDmode, the n-channel transistor 1001 is switched on and the p-channeltransistor 1002 is switched off. In this way, the output of the linearamplifier 1004 is strongly coupled to ground, thereby emulating theswitch of earlier examples. Although this example is described in termsof switching to ground via n-channel transistor 1001 (e.g. an nMOSswitch), a more general case may be to switch to a DC voltage, whichcould be a supply voltage, in which case the p-channel transistor 1002could be used. By encompassing the amplification and ground switchingfunctions in a single element, the complexity of the circuit can beadvantageously reduced.

In one alternative implementation, the NMOS device(s) of the linearamplifier 1004 may be used together with a supplementary switch (notshown), in order to use and benefit from a combination of both methodsand architecture of FIG. 8 and FIG. 10.

FIG. 11 illustrates a yet still even further example block diagram 1100of a part of a power supply circuit of for a PA of a transmitter chainof a wireless communication unit, adapted to support envelope tracking.The example of FIG. 11 illustrates an alternate way to implement thecontrol loop for ET mode in which the FD control loop and ET controlloop are largely independent of each other. In some examples, this maybe advantageous if different characteristics are desired for the twocontrol loops. For ease of understanding, and not to obfuscate ordistract from the description of FIG. 11, electronic components andcircuits of the transmitter chain described with reference to earlierfigures will not be explained in any greater extent than that considerednecessary.

The ET mode control loop of the circuit of FIG. 11 contains aproportional integral (PI) controller 1101 that is independent from thelow-frequency-path regulator. Within the PI controller 1101 there is acurrent-voltage (I/V) converter 1102 that converts the currentinformation from current sensor 901 into a voltage. There is also adifference amplifier 1103 that amplifies the difference between theoutput voltage 512 of the linear amplifier 504 and a reference voltageREF_ET 1104. The reference voltage REF_ET 1104 represents the desiredaverage output voltage 512 of the linear amplifier 504. The output ofthe difference amplifier 1103 is integrated by the integrator 1105. Thedifference between the output voltage of the integrator 1105 and that ofthe I/V converter 1102 is then computed by the summation circuit 1106.

The output voltage 1110 of the summation circuit 1106 represents theinstantaneous current at the output of the linear amplifier 504, plus aslowly-varying term that reflects the integrated difference between theoutput voltage 512 of the linear amplifier 504 and its desired value1104. As such, the output voltage 1110 can be used directly as thecontrol voltage to a pulsewidth modulator in order to produce theappropriate current to apply to the coupling network. The analogmultiplexer 1107 passes this voltage 1110 through to the comparator 630,which compares it to a fixed voltage THR_ET 1108, passed through analogmultiplexer 1109. The voltage at THR_ET 1108 can be chosen to null outany offsets in the control loop, for example a finite offset resultingfrom the current ripple through the inductor 622. The resulting PWMwaveform is used to control the current through the inductor 622, as inthe other example embodiments.

In closed-loop operation, the ET mode control loop tends to force theaverage output voltage 512 of the linear amplifier 504 to be equal tothe desired value 1104. It also tends to force the instantaneous outputcurrent of the linear amplifier to zero by supplying more currentthrough the low-frequency path when the output current of the linearamplifier is high.

The FD mode control loop of the circuit of FIG. 11 operates the same asin FIG. 8. The only difference is the location in the circuit of theanalog multiplexers 1107 and 1109.

In such ET architectures, the integrity of the peaks of the envelopewaveform must be maintained, whereas the integrity of the troughs of theenvelope waveform is not critical, provided sufficient voltage headroomis maintained. The troughs of the waveform are associated with a highvoltage slew rate. Therefore, in some examples of the invention, themodulated power supply 528 provided to the PA 424 may be referenced to amodified envelope waveform, with the troughs of the envelope waveformclipped or removed, i.e. the depths of the envelope waveform troughs arereduced. Removing the troughs reduces the high-frequency components fromthe voltage waveform, whilst increasing the DC content of the voltagewaveform. This concept will be hereinafter termed ‘de-troughing’.

The envelope waveform troughs correspond to the periods of minimumoutput power from PA 424 and, thus, the clipping or removal(de-troughing) of the troughs of the envelope waveform has minimal (orat least reduced) impact on overall PA power dissipation. In effect, theoperating region of the PA 424 results in the PA 424 exhibitingcharacteristics of a current sink rather than a resistor, with thecurrent being a function of the instantaneous power. The power drawn bythe linear amplifier 504 will be I_(ac)V_(amp) where V_(amp) is theamplifier supply voltage.

De-troughing increases the power dissipated by the PA 424, since thecurrent supplied to the PA 424 is essentially the same, but where thevoltage at the supply port of the PA is increased. However, sincede-troughing is applied at the points of lowest output power, the impactis minimal. De-troughing the reference waveform also reduces thepeak-to-peak voltage associated with the high frequency path, therebyreducing the amplifier supply requirements and improving the overallefficiency via the use of a second SMPS.

Thus, and advantageously in an AC coupled architecture as illustrated,de-troughing a waveform has the effect of reducing the peak-to-peakvalue (AC content), whilst increasing the DC value, and thereby thesupply requirement of the linear amplifier 504 is reduced. In effect,additional efficiency in PA power supply may be achieved from theincreased efficiency of the low-frequency supply path, as the reductionin voltage supplied from the high-frequency supply amplifier has beeneffectively, and favourably traded for increased low-frequency energyfrom the more efficient SMPS.

In one example embodiment, the envelope signal 602 applied to the linearamplifier 504 may be pre-conditioned by de-troughing, in order to reducethe envelope signal headroom with little or minimal impact on the RFperformance of the PA 424. In some examples, the pre-conditioning byde-troughing may involve a procedure as simple as limiting a minimumvalue of the reference waveform to a fixed value, such as the minimumvoltage requirement of the PA load. Alternatively, in other examples,the minimum value may be related to the average or rms value of theenvelope waveform (e.g. 9 dB below the rms value). In one exampleembodiment, the de-troughing of the envelope signal 602 applied to thelinear amplifier 504 may be additionally pre-conditioned by, sayfiltering.

FIG. 12 illustrates a simplified example flowchart 1200 to supportenvelope tracking (ET) in a transmitter chain. The flowchart starts instep 1205 with, say, the transmitter commencing a power level updateprocess. The transmitter starts to modulate signals for transmissionusing, say, a pre-determined modulation scheme in step 1210 and sets aninitial radio frequency output power level of the transmitter in step1215. A determination is then made as to whether envelope tracking isrequired, as shown in step 1220. If envelope tracking is beneficial orrequired in step 1220, then a determination is made as to whether thecurrent mode being used is envelope tracking, as in step 1225. If thecurrent mode of operation is envelope tracking in step 1225, one or moremodulator parameters are adjusted within the transmitter chain, in step1230, an ET to ET transition is performed in step 1235 and the processends in step 1240.

However, if the current mode of operation is not envelope tracking, instep 1225, one or more modulator parameters are adjusted within thetransmitter chain, in step 1250, a FD to ET transition is performed instep 1265, disabling FD mode and enabling ET mode, and the process endsin step 1240.

Referring back to step 1220, if the mode of operation required is notenvelope tracking, a determination is made as to whether the currentmode of operation is FD, as shown in step 1245. If the current mode ofoperation is FD, in step 1245, one or more modulator parameters areadjusted within the transmitter chain, in step 1250, a FD to FDtransition is performed in step 1255 and the process ends in step 1240.However, if the current mode of operation is not FD, the modulatorparameters are adjusted within the transmitter chain, in step 1270.Thereafter an ET to FD transition in step 1275 is performed, causing theenvelope tracking mode of operation to be disabled and the fixed drainmode enabled, in step 1275 and the process ends in step 1240.

In some examples, some or all of the steps illustrated in the flowchartmay be implemented in hardware and/or some or all of the stepsillustrated in the flowchart may be implemented in software. In someexamples, the aforementioned steps of FIG. 12 may be re-ordered, whilstproviding the same or similar benefits.

Thus, the hereinbefore examples provide improved power supply integratedcircuits, wireless communication units and methods for power amplifiersupply voltage control that use such linear and efficient transmitterarchitectures, and in particular a wideband power supply architecturethat can provide a supply voltage in power efficient manner.Advantageously, example embodiments of the invention based on an ACcoupled architecture may provide improved efficiency over DC coupledsolutions. For example, in a dc-coupled system where the output of thelinear amplifier is directly connected to the PA supply (i.e. the outputof the modulator), the output cannot exceed the supply of the linearamplifier without forward biasing diodes associated with the outputdevices. However, in an AC coupled system as described, the capacitor isan additional component, with an associated cost. However, the provisionof two modes of operation, in the various architectures described,supports a dual-role of the coupling capacitor. The architectures allowfor the coupling capacitor to function both as the filtering capacitorfor the SMPS in fixed drain mode and as an AC coupling capacitor inenvelope tracking mode.

Advantageously, some of the example embodiments of the invention mayalso provide an ability to drive loads above the power source voltage(Vbat). For example, the linear amplifier may be implemented withgreater than unity gain, which allows output voltages greater thanbattery voltage to be mapped to inputs less than the battery voltage.The dc (average) output voltage, which is set by the compliance of theLF supply SMPS, is limited to voltages less than the battery voltage ifa buck type regulator is used. However the output of the modulator isthe combination (addition) of the DC and AC components. Positive ACvoltages, applied at the output of the amplifier, will drive the outputhigher above the average level, i.e. above the battery voltage. Thisonly works in a transient manner, the DC voltages still remains belowbattery voltage, and relies on the capacitor's ability to maintain a DCvoltage across and act as a level shifter. The presence of an inductorbetween the output of the modulator and switching devices of the LFSupply SMPS is necessary to enable the voltage to momentarily exceed thebattery voltage.

Advantageously, some example embodiments of the invention provide anability to switch between an ET mode of operation and a FD mode ofoperation, dependent upon the prevailing operational conditions. Inparticular, an ability to reconfigure a SMPS power supply from an ETmode of operation to a fixed drain mode of operation, at least for aperiod of time, may negate or reduce a capacitance cost of thearchitecture, as the AC-coupling capacitor may be re-used in an FD modeof operation as a filter capacitor.

In some further example embodiments, it is envisaged that a furthercombination of ET and FD modes may be supported for PA power control,for example dependent upon output power level. For example, such anapproach in switching between modes of operation may be employed forscenarios when a FD mode of operation may be predominantly used forlower output power levels and where this is transitioned to an ET modeof operation when the output power is required to approach its maximumlevel e.g. when the PA output power approaches its top 10 dB of outputpower range. In this example, therefore, two distinct ET modulators maybe used. A first ET modulator may be configurable/re-configurable as anormal DC-DC supply to provide the supply for the final (higher power)output stage. A second ET modulator (separate DC-DC supply) may be usedto supply lower output power stages, e.g. for at least one amplifierstage prior to the RF PA output stage.

However, in some examples, such a dual DC-DC supply approach may beconsidered as being not cost-effective. In some cases, therefore, in atwo separate supply scenario, the lower output power stages (e.g. atleast one amplifier stage prior to the RF PA output stage) may use adifferent (first) supply (say VCC1) that is connected to the battery ora linear supply. In such an implementation, however, best powerefficiency is not achieved. The other stages of the transmit chain (e.g.at least one amplifier stage prior to the RF PA output stage) may besupplied with a second supply, say an average power tracking (APT) DC-DCsupply.

In some further alternative example embodiments, ET may be applied toall stages of the PA when a high output power is required, in contrastto the usual method of only applying ET to the final output stage.However, by applying ET to all stages of the PA, there is a consequentpotential impact on performance due to, for example, bias networklimitations, noise performance or linearity considerations.

In some further example embodiments, when the output stage of the PA isapplied with an envelope tracking supply, the other stages of thetransmit chain (e.g. (at least one) amplifier stage prior to the RF PAoutput stage) may be supplied with an average power tracking (APT) DC-DCsupply. In some examples, such an average power tracking DC-DC supplymay also be the power supply for the AC (high frequency) path of thewideband envelope modulator. As illustrated in FIG. 15, the control ofthe supply to the other stages of the transmit chain (e.g. (at leastone) amplifier stage prior to the RF PA output stage) may be configuredvia a switch, in turn controlled by a mode control module.

In some further example embodiments, if an ET mode of operation cannotbe used for some reason, it is envisaged that an average power trackingsupply may be used to reduce the impact caused by the first stage of thePA on the overall power dissipation.

In some further example embodiments, one or more of the aboveembodiments may be achieved using a further DC-DC supply to ensure anoptimal operation of the PA. However, in some example embodiments, toavoid the additional cost of using a further DC-DC supply, analternative circuit architecture may be employed, as illustrated in FIG.13.

Referring now to FIG. 13, a further example circuit diagram 1300 of apart of a power amplifier circuit of a transmitter chain of a wirelesscommunication unit, for example a radio frequency integrated circuit1395, is illustrated. The RF integrated circuit 1395 comprises a radiofrequency (RF) power amplifier (PA) output stage and at least oneamplifier stage prior to the RF PA output stage. The RF integratedcircuit 1395 may comprise or be operably coupled to a linear amplifiercomprising a voltage feedback wherein the linear amplifier is operablycoupled to a low frequency supply module such that the linear amplifierand low frequency supply module provide a combined power supply to theRF PA output stage, as described in other example embodiments. The RFintegrated circuit 1395 may also comprise or be operably coupled to aswitched mode power supply module arranged to provide a power supply tothe linear amplifier and to the at least one amplifier stage prior tothe RF PA output stage.

In this manner, the further example circuit diagram 1300 may be arrangedto support both envelope tracking and fixed drain modes of operation.FIG. 13 illustrates a two-stage PA, with an amplifier stage 1325 (priorto the RF PA output stage 1324) being biased by a first bias network1320 and receiving input signal 1315, which in some example embodimentsis a full transmit signal on a radio frequency carrier for radiatingfrom an antenna. The amplifier stage 1325 prior to the RF PA outputstage 1324 is supplied by a first supply voltage (VCC1) 1305. The outputof the amplifier stage 1325 prior to the RF PA output stage 1324 is theninput to a RF PA output stage 1324 of the two-stage PA via a firstmatching network 1330. The RF PA output stage 1324 is supplied by asecond supply voltage (VCC2) 1310 and is biased by a second bias network1328. The output of the RF PA output stage 1324 is operably coupled to asecond matching network 1335.

Although FIG. 13 illustrates a two-stage PA, it is envisaged in otherexample embodiments that an alternative number of stages may be used,for example a three-stage or four-stage PA with a plurality of amplifierstages prior to the RF PA output stage 1324.

In some examples, the second supply voltage (VCC2) 1310 may be employedfor an ET mode of operation, for example to achieve higher power levelsfor the PA output. In some examples, the second supply voltage (VCC2)1310 may be employed using an FD mode of operation, for example whenlower power levels for the PA output are required. The supply for eitherthe ET mode of operation or the FD mode of operation can be implementedusing any of the techniques or circuits highlighted in FIGS. 5-6 orFIGS. 8-11. Similarly, in some examples, a first supply voltage (VCC1)1305 may be employed for an ET mode of operation, for example to achievehigher output power levels for the amplifier stage 1325 prior to the RFPA output stage 1324. In some examples, the first supply voltage (VCC1)1305 may alternatively be employed using an FD mode of operation, forexample when lower output power levels for the amplifier stage 1325prior to the RF PA output stage 1324 are required. Thus, in someexamples, the first supply voltage (VCC1) 1305 may support a differentmode of operation to the second supply voltage (VCC2) 1310. In someexamples, the first supply voltage (VCC1) 1305 may support the same modeof operation to the second supply voltage (VCC2) 1310.

In some examples, different values may be used in the ET and FD modes ofoperation, across the power amplifier stages, including the amplifierstage 1325 prior to the RF PA output stage 1324 and RF PA output stage1324. In some examples, the values to be employed in either ET or FDmodes of operation for either the amplifier stage 1325 prior to the RFPA output stage 1324 or the RF PA output stage 1324 may be controlledusing different mapping tables, for example in order to achieve improved(or ideally optimum) efficiency.

In such a manner, a close to optimal supply voltage configuration may beprovided to the power amplifier over all desired output power levels,for example by enabling/disabling an ET mode of operation orenabling/disabling a FD mode of operation and/or transitioning therebetween, (for example via a switch configuration shown in FIG. 14).

In some examples, the first supply voltage 1305 may be a DC-DC supply,which is arranged to provide a decreasing supply voltage in response toa decreasing output power required from the amplifier stage 1325 priorto the RF PA output stage 1324. In some examples, this decrease betweenthe DC-DC supply voltage and the decreasing power level at the output ofthe RF PA output stage 1324 may not be a linear relationship.

Referring now to FIG. 14, a further simplified example circuit diagram1400 of a part of a power supply circuit of a transmitter chain of awireless communication unit, for example a radio frequency integratedcircuit 1495, is illustrated. In this example, the further simplifiedexample circuit diagram 1400 may be again adapted to support bothenvelope tracking and fixed drain modes of operation, in a similarmanner to that described with respect to FIG. 13.

Thus, FIG. 14 again illustrates a two-stage PA, with at least oneamplifier stage 1425 prior to the RF PA output stage 1424 receivinginput signal 1415. However, in this example, the at least one amplifierstage 1425 prior to the RF PA output stage 1424 is supplied by a firstsupply voltage 1405, if a switch 1414 (controlled by mode control switch1416) is operably coupled/configured to node/position ‘A’. In thisexample, the at least one amplifier stage 1425 prior to the RF PA outputstage 1424 may alternatively be supplied by a second supply voltage1410, if the switch 1414 (controlled by mode control switch 1416) isoperably coupled/configured to node/position ‘B’. In this example, theRF PA output stage 1424 is supplied by the second supply voltage 1410,irrespective of the configuration of the switch 1414.

The full transmit signal output from the (at least one) amplifier stage1425 prior to the RF PA output stage 1424 is then input to the RF PAoutput stage 1424 of the two-stage PA via a first matching network, forexample capacitor 1430. The RF PA output stage 1424 is supplied by asecond supply voltage (VCC2) 1410. The output of the RF PA output stage1424 is operably coupled to a second matching network 1435. Inaccordance with example embodiments, and in the same manner as previousexamples, the first supply and second supply are coupled to the at leastone amplifier stage 1425 prior to the RF PA output stage 1424 and the RFPA output stage 1424 by respective L-C networks 1440, 1445. Inaccordance with example embodiments, and in the same manner as FIG. 13,bias network 1420 provides a (first) suitable bias voltage for theamplifier stage 1425 prior to the RF PA output stage 1424 and biasnetwork 1428 provides a (second) suitable bias voltage for the RF PAoutput stage 1424 of the two-stage PA.

Although FIG. 14 illustrates a two-stage PA, it is envisaged in otherexample embodiments that an alternative number of stages may be used,for example a three-stage or four-stage PA.

In accordance with some examples, mode control switch 1414 may be asingle pole double throw (SPDT) switch, as shown, that is controlled bymode control module 1416 to switch the supply provided to the (at leastone) amplifier stage 1425 prior to the RF PA output stage 1424 betweenthe first supply voltage 1405 and the second supply voltage 1410. Whenimplemented with a selectable ET and/or FD supply option for the firstsupply voltage 1405 and the second supply voltage 1410, as described atleast with respect to FIG. 13, the at least one amplifier stage 1425prior to the RF PA output stage 1424 may be selectable to receive oneof: a combined first power supply (e.g. the output from both a linearamplifier and a low frequency supply module), or the second power supplyfrom the switched mode power supply module. As shown, the switch 1414 iscontrolled by a mode control module 1416 operably coupled to the switch1414 and arranged to control a supply path through the switch that mayselect between at least an envelope tracking mode of operation and afixed drain mode of operation of the at least one amplifier stage 1425prior to the RF PA output stage 1424. In some examples, the mode controlmodule 1416 may select between at least an envelope tracking mode ofoperation and a fixed drain mode of operation to be applied to the atleast one amplifier stage 1425 prior to the RF PA output stage 1424based on a power output level of the RF PA output stage 1424. In someexamples, the mode control module 1416 may select between the envelopetracking mode of operation and the fixed drain mode of operation of theat least one amplifier stage 1425 prior to the RF PA output stage 1424based on at least one from a group of: an output power level of the RFPA output stage 1424, use of a modulation scheme that has a low crestfactor, etc., as previously described.

In this further simplified example circuit diagram 1400, it isnoteworthy that this (mode control) switch 1414 is distinct fromprevious switches, e.g. switch 614 from FIG. 6, in that this is a secondswitch that is used to provide a selectable supply to the (at least one)amplifier stage 1425 prior to the RF PA output stage 1424 of, say, atwo-stage (or multiple stage) PA.

Again, in some examples, the second supply voltage 1410 may beconfigured to provide for an ET mode of operation, for example toachieve higher power levels for the PA output and more importantlyhigher efficiency of operation. Alternatively, in some examples, thesecond supply voltage 1410 may be employed using an FD mode ofoperation, for example for lower power levels for the PA output, i.e.both for the RF PA output stage 1424 and for the (at least one)amplifier stage 1425 prior to the RF PA output stage 1424 (when theswitch 1414 (controlled by mode control switch 1416) is operablycoupled/configured to node/position ‘B’). Alternatively, in someexamples, the first supply voltage 1405 may be arranged to supply an FDmode of operation to the amplifier stage 1425 prior to the RF PA outputstage 1424, for example when lower output power levels for saidamplifier stage(s) is/are required, whilst the second supply voltage1410 may be configured to provide for an ET mode of operation for the RFPA output stage 1424.

In such a manner, a close to optimal supply voltage configuration may beprovided to the power amplifier over all desired output power levels,for example by enabling/disabling an ET mode of operation orenabling/disabling a FD mode of operation and transitioning therebetween, (for example via switch 1414).

Although other examples are described with reference to a specificmodulator implementation (e.g. a dual HF and LF supply modulearrangement), it is envisaged that, in some examples, the concept hereindescribed may be employed in other modulator architectures where asecondary supply source is used.

Referring now to FIG. 15, a yet further example circuit diagram 1500 ofa part of a power supply circuit of a transmitter chain of a wirelesscommunication unit, for example a radio frequency integrated circuit1595, is illustrated. The yet further example circuit diagram 1500 mayalso be adapted to support both envelope tracking and fixed drain modesof operation, as described. The yet further example circuit diagram 1500comprises a number of elements and components corresponding to earlierexample embodiments, and as such these will not be described in anygreater detail than required, in order to not obfuscate the description.

Again, FIG. 15 illustrates a two-stage PA, with at least one amplifierstage 1525 prior to the RF PA output stage 1524 being supplied by afirst supply voltage, for example from a switched mode power supplymodule, e.g. high frequency supply module 1520 arranged to provide apower supply to the linear amplifier 1504 and to the at least oneamplifier stage 1525 prior to the RF PA output stage 1524 when theswitch 1514 is coupled to Node ‘A’ 1505.

Alternatively, the at least one amplifier stage 1525 prior to the RF PAoutput stage 1524 may be supplied by a second supply voltage, forexample from a combination of the output from the linear amplifier 1504and the low frequency supply module 1518 when the switch 1514 is coupledto Node ‘B’ 1510. The full transmit signal output of the at least oneamplifier stage 1525 prior to the RF PA output stage 1524 is input tothe RF PA output stage 1524 of the two-stage PA. The RF PA output stage1524 is biased by bias network 1528 and supplied by the second supplyvoltage 1510, which is a combination of the output from the linearamplifier 1504 and the low frequency supply module 1518.

Thus, in the illustrated example, the at least one amplifier stage 1525prior to the RF PA output stage 1524 may be supplied by the first supplyvoltage 1520, say from a HF path supply module 1506 of the ET modulator.Alternatively, as shown, the at least one amplifier stage 1525 prior tothe RF PA output stage 1524 may be supplied by an ET signal identifiedas second supply voltage routed via Node ‘B’ 1510 of switch 1514. Inthis example, the ET signal identified as the second supply voltage,supplied to at least the RF PA output stage 1524, is the combined ETmodulator signal as a whole (comprising the output from low frequencysupply module 1518 and the output from linear amplifier 1504).Therefore, in this example, envelope tracking is applied to both the atleast one amplifier stage 1525 prior to the RF PA output stage 1524 andthe RF PA output stage 1524.

In some examples, the linear amplifier 1504 may comprise anumber/selection of inputs, e.g. a reference input signal 1503 to setthe PA supply for an ET mode of operation, for example to achieve higherpower levels, and a DC input to supply the RF PA output stage 1524 for aFD mode of operation, for example to perform at lower power levels. Inthis manner, the supply provided HF supply 1506 may be configured asfixed DC or may be configured to amplify an envelope signal, which maybe different to the envelope signal applied on the main (AC) signal pathrouted through the PA. By applying an envelope tracking signal (from ahigh efficiency power supply) to the linear amplifier 1504, an overallhigher power efficiency may be achieved.

Thus, in some examples, two distinct envelope tracking signals may beused as supplies, a first ET supply signal applied solely to the atleast one amplifier stage 1525 prior to the RF PA output stage 1524 anda second ET supply signal applied solely to the RF PA output stage 1524.In some examples, the two envelope tracking signals may be shaped (e.g.mapped) differently when ET is to be applied to improve linearity orefficiency. In this manner, and advantageously, each path can beindependently optimized. For example, a first envelope signal applied bya combination of the linear amplifier 1504 and low frequency supplymodule 1518 to the RF PA output stage 1524 and a second (different)envelope supply signal applied to the at least one amplifier stage 1525prior to the RF PA output stage 1524 via switched mode supply 1506) maybe configured to be different in one or more of the followingcharacteristics: different frequency response (where a differentfrequency response may exhibit an improved performance with respect topre-emphasis or de-emphasis), provide a different delay, provide fordifferent pre-conditioning of peaks and troughs, etc. In any of theabove example embodiments, it is envisaged that various circuitparameters may be individually controlled using values from one or moremapping tables (not shown), in order to improve efficiency.

In this manner, a close-to-optimal supply voltage configuration may beprovided to the PA (comprising at least the at least one amplifier stage1525 prior to the RF PA output stage 1524 and the RF PA output stage1524) over all desired output power levels, for example byenabling/disabling an ET mode of operation or enabling/disabling a FDmode of operation and transitioning there between, (for example viaswitch 1514 under control of mode control module 1516). As shown, theswitch 1514 is controlled by a mode control module 1516 operably coupledto the switch 1514 and arranged to control a supply path through theswitch that may select between at least an envelope tracking mode ofoperation and a fixed drain mode of operation of the at least oneamplifier stage 1525 prior to the RF PA output stage 1524. In someexamples, the mode control module 1516 may select between at least anenvelope tracking mode of operation and a fixed drain mode of operationto be applied to the at least one amplifier stage 1525 prior to the RFPA output stage 1524 based on a power output level of the RF PA outputstage 1524. In some examples, the mode control module 1516 may beoperably coupled to a second switch 514 and arranged to additionallyselect between at least an envelope tracking mode of operation and afixed drain mode of operation to be applied to the RF PA output stage1524.

Thus, in the example embodiment in FIG. 15, as in earlier drawings, a HFsupply module forms part of an ET supply modulator, in order to beeither a fixed supply (from a DC-DC converter) or provide an ET supplyto linear amplifier 1504. The output of the linear amplifier 1504, incombination with the output of the LF supply module 1518 serves as thesupply for the RF PA output stage 1524. Optionally, the output of thelinear amplifier 1504, in combination with the output of the LF supplymodule 1518 may also serve as the supply for at least one amplifierstage 1525 prior to the RF PA output stage 1524. In this exampleembodiment, it is envisaged that the at least one amplifier stage priorto the RF PA output stage 1525 and RF PA output stage 1524 may operateat similar voltage levels, whereby a similar reduction in the supplyvoltage may be applied to both the at least one amplifier stage 1525prior to the RF PA output stage 1524 and the RF PA output stage 1524 asthe power output is reduced.

Although this example is described with reference to a specific dual HFand LF supply module arrangement modulator implementation, it isenvisaged that, in other examples, the concept herein described may beemployed in other modulator architectures where a secondary supplysource is used that is not necessarily based on a dual HF and LF supplymodule arrangement. Although FIG. 15 illustrates a two-stage PA, it isenvisaged in other example embodiments that an alternative number ofstages may be used, for example a three-stage or four-stage PA.

Referring now to FIG. 16, there is illustrated a typical computingsystem 1600 that may be employed to implement signal processingfunctionality in embodiments of the invention. Computing systems of thistype may be used in access points and wireless communication units.Those skilled in the relevant art will also recognize how to implementthe invention using other computer systems or architectures. Computingsystem 1600 may represent, for example, any general purpose computingdevice as may be desirable or appropriate for a given application orenvironment. Computing system 1600 can include one or more processors,such as a processor 1604. Processor 1604 can be implemented using ageneral or special-purpose processing engine such as, for example, amicroprocessor, microcontroller or other control module. In thisexample, processor 1604 is connected to a bus 1602 or othercommunications medium.

Computing system 1600 can also include a main memory 1608, such asrandom access memory (RAM) or other dynamic memory, for storinginformation and instructions to be executed by processor 1604. Mainmemory 1608 also may be used for storing temporary variables or otherintermediate information during execution of instructions to be executedby processor 1604. Computing system 1600 may likewise include a readonly memory (ROM) or other static storage device coupled to bus 1602 forstoring static information and instructions for processor 1604.

The computing system 1600 may also include information storage system1610, which may include, for example, a media drive 1612 and a removablestorage interface 1620. The media drive 1612 may include a drive orother mechanism to support fixed or removable storage media, such as ahard disk drive, a floppy disk drive, a magnetic tape drive, an opticaldisk drive, a compact disc (CD) or digital video drive (DVD) read orwrite drive (R or RW), or other removable or fixed media drive. Storagemedia 1618 may include, for example, a hard disk, floppy disk, magnetictape, optical disk, CD or DVD, or other fixed or removable medium thatis read by and written to by media drive 1612. As these examplesillustrate, the storage media 1618 may include a computer-readablestorage medium having particular computer software or data storedtherein.

In alternative embodiments, information storage system 1610 may includeother similar components for allowing computer programs or otherinstructions or data to be loaded into computing system 1600. Suchcomponents may include, for example, a removable storage unit 1622 andan interface 1620, such as a program cartridge and cartridge interface,a removable memory (for example, a flash memory or other removablememory module) and memory slot, and other removable storage units 1622and interfaces 1620 that allow software and data to be transferred fromthe removable storage unit 1618 to computing system 1600.

Computing system 1600 can also include a communications interface 1624.Communications interface 1624 can be used to allow software and data tobe transferred between computing system 1600 and external devices.Examples of communications interface 1624 can include a modem, a networkinterface (such as an Ethernet or other NIC card), a communications port(such as for example, a universal serial bus (USB) port), a PCMCIA slotand card, etc. Software and data transferred via communicationsinterface 1624 are in the form of signals which can be electronic,electromagnetic, and optical or other signals capable of being receivedby communications interface 1624. These signals are provided tocommunications interface 1624 via a channel 1628. This channel 1628 maycarry signals and may be implemented using a wireless medium, wire orcable, fiber optics, or other communications medium. Some examples of achannel include a phone line, a cellular phone link, an RF link, anetwork interface, a local or wide area network, and othercommunications channels.

In this document, the terms ‘computer program product’,‘computer-readable medium’ and the like may be used generally to referto media such as, for example, memory 1608, storage device 1618, orstorage unit 1622. These and other forms of computer-readable media maystore one or more instructions for use by processor 1604, to cause theprocessor to perform specified operations. Such instructions, generallyreferred to as ‘computer program code’ (which may be grouped in the formof computer programs or other groupings), when executed, enable thecomputing system 1600 to perform functions of embodiments of the presentinvention. Note that the code may directly cause the processor toperform specified operations, be compiled to do so, and/or be combinedwith other software, hardware, and/or firmware elements (e.g., librariesfor performing standard functions) to do so.

In an embodiment where the elements are implemented using software, thesoftware may be stored in a computer-readable medium and loaded intocomputing system 1600 using, for example, removable storage drive 1622,drive 1612 or communications interface 1624. The control module (in thisexample, software instructions or computer program code), when executedby the processor 1604, causes the processor 1604 to perform thefunctions of the invention as described herein.

In particular, it is envisaged that the aforementioned inventive conceptcan be applied by a semiconductor manufacturer to any integrated circuitcomprising a power supply circuit for a PA. It is further envisagedthat, for example, a semiconductor manufacturer may employ the inventiveconcept in a design of a stand-alone device, such as a power supplymodule, or application-specific integrated circuit (ASIC) and/or anyother sub-system element. Alternatively, the examples of the inventionmay be embodied in discrete circuits or combination of components.

It will be appreciated that, for clarity purposes, the above descriptionhas described embodiments of the invention with reference to differentfunctional units and processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits or processors, for example with respect to the power supplycircuitry or signal conditioning circuits or amplifier circuits may beused without detracting from the invention. For example, functionalityillustrated to be performed by separate processors or controllers may beperformed by the same processor or controller. Hence, references tospecific functional units are only to be seen as references to suitablemeans for providing the described functionality, rather than indicativeof a strict logical or physical structure or organization.

Aspects of the invention may be implemented in any suitable formincluding hardware, software, firmware or any combination of these. Theinvention may optionally be implemented, at least partly, as computersoftware running on one or more data processors and/or digital signalprocessors or configurable module components such as field programmablegate array (FPGA) devices. Thus, the elements and components of anembodiment of the invention may be physically, functionally andlogically implemented in any suitable way. Indeed, the functionality maybe implemented in a single unit, in a plurality of units or as part ofother functional units.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term ‘comprising’ does not exclude the presence ofother elements or steps.

Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by, for example, a singleunit or processor. Additionally, although individual features may beincluded in different claims, these may possibly be advantageouslycombined, and the inclusion in different claims does not imply that acombination of features is not feasible and/or advantageous. Also, theinclusion of a feature in one category of claims does not imply alimitation to this category, but rather indicates that the feature isequally applicable to other claim categories, as appropriate.

Furthermore, the order of features in the claims does not imply anyspecific order in which the features must be performed and in particularthe order of individual steps in a method claim does not imply that thesteps must be performed in this order. Rather, the steps may beperformed in any suitable order. In addition, singular references do notexclude a plurality. Thus, references to ‘a’, ‘an’, ‘first’, ‘second’,etc. do not preclude a plurality.

Thus, an improved power supply integrated circuit(s), wirelesscommunication unit(s) and methods for power amplifier supply voltagecontrol that use linear and efficient transmitter architectures, and inparticular a wideband power supply architecture that can provide asupply voltage in power efficient manner therefor, have been described,wherein the aforementioned disadvantages with prior art arrangementshave been substantially alleviated.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. An integrated circuit comprising: a radiofrequency (RF) power amplifier (PA) output stage; at least one amplifierstage prior to the RF PA output stage; a linear amplifier comprising avoltage feedback wherein the linear amplifier is operably coupled to alow frequency supply module such that the linear amplifier and lowfrequency supply module provide a combined power supply to the RF PAoutput stage; and a switched mode power supply module arranged toprovide a power supply to the linear amplifier and to the at least oneamplifier stage prior to the RF PA output stage.
 2. The integratedcircuit of claim 1 further comprising a first switch operably coupled tothe at least one amplifier stage prior to the RF PA output stage andselectable to receive one of: the combined first power supply, thesecond power supply from the switched mode power supply module.
 3. Theintegrated circuit of claim 2 further comprising a mode control moduleoperably coupled to the first switch and arranged to control a supplypath through the first switch and select between at least an envelopetracking mode of operation and a fixed drain mode of operation of the atleast one amplifier stage prior to the RF PA output stage.
 4. Theintegrated circuit of claim 3 wherein the mode control module isarranged to select between at least an envelope tracking mode ofoperation and a fixed drain mode of operation to be applied to the atleast one amplifier stage prior to the RF PA output stage based on apower output level of the RF PA output stage.
 5. The integrated circuitof claim 3 wherein the mode control module is operably coupled to asecond switch and arranged to additionally select between at least anenvelope tracking mode of operation and a fixed drain mode of operationto be applied to the RF PA output stage.
 6. The integrated circuit ofclaim 5 wherein the mode control module is arranged to select between atleast an envelope tracking mode of operation and a fixed drain mode ofoperation to be applied to the RF PA output stage based on a poweroutput level of the RF PA output stage.
 7. The integrated circuit ofclaim 6 wherein the mode control module is arranged to: switch a supplyto the at least one amplifier stage prior to the RF PA output stage tothe envelope tracking mode of operation for a high power output level ofthe RF PA output stage; and switch a supply to the fixed drain mode ofoperation for a low power output level of the RF PA output stage.
 8. Theintegrated circuit of claim 4 wherein the mode control module isarranged to select between the envelope tracking mode of operation andthe fixed drain mode of operation of the at least one amplifier stageprior to the RF PA output stage based on at least one from a group of:an output power level of the RF PA output stage, use of a modulationscheme that has a low crest factor.
 9. The integrated circuit of claim 1wherein the linear amplifier comprises a first input for receiving thevoltage feedback and a second input arranged to receive a firstmodulated envelope input signal.
 10. The integrated circuit of claim 9wherein the combined first power supply to the RF PA output stage basedon the first modulated envelope input signal is arranged to be differentto a second envelope modulated signal applied to the linear amplifierand the at least one amplifier stage prior to the RF PA output stage.11. The integrated circuit of claim 10 wherein the second modulatedenvelope signal is different to the first envelope modulated signal inat least one of the following characteristics: different frequencyresponse, different delay, different pre-conditioning of peaks andtroughs.
 12. A wireless communication unit comprising: a radio frequency(RF) power amplifier (PA) output stage; at least one amplifier stageprior to the RF PA output stage; a linear amplifier comprising a voltagefeedback wherein the linear amplifier is operably coupled to a lowfrequency supply module such that the linear amplifier and low frequencysupply module provide a combined first power supply to the RF PA outputstage; and a switched mode power supply module arranged to provide asecond power supply to the linear amplifier and to the at least oneamplifier stage prior to the RF PA output stage.
 13. A method forsupplying power in a transmitter comprising a RF PA output stage, atleast one amplifier stage prior to the RF PA output stage, a switchedmode power supply module, a linear amplifier comprising a voltagefeedback and a low frequency supply module operably coupled to thelinear amplifier, the method comprising: providing a combined firstpower supply to the RF PA output stage from both the linear amplifierand the low frequency supply module; and providing a second power supplyto both the at least one amplifier stage prior to the RF PA output stageand the linear amplifier from the switched mode power supply module.